Prof. Dr.-Ing. Stefan Deßloch AG Heterogene Informationssysteme Geb. 36, Raum 329 Tel. 0631/205 3275 [email protected] Chapter 10 Web-based Information Systems Role of the WWW for IS n Initial purpose: sharing information on the internet n technologies n HTML documents n HTTP protocol n web browser as client for internet information access n For Information Systems: connecting remote clients with applications across the internet/intranet n "web-enabled" applications n extend application reach to the consumer n leverage advantages of web technologies n web browser as a universal application client n "thin client" n no application-specific client code has to be installed n requirements n content is coming from dynamic sources (IS, DBS) n request to access a resource has to result in application invocation n session state: tracking repeated interactions of the same client with a web server 2 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Architecture for Web-IS client n Presentation layer may be realized in separate tiers web browser n client-side presentation using browser, client components (optional) presentation web server n server-side presentation done by web layer server, dynamic HTML generation information system information (HTML filter) HTML filter n Presentation components interact with application logic components n managed by appl. server, or run application (or web) server within web server environment application logic n Access to RM layer n "encapsulated" in appl. logic component n may also be performed directly within resource management presentation logic component layer n special case, if there is no application logic (only data access logic) 3 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Overview persistent (applet) HTML: HyperText Markup Language Web Browser program cache CGI: Common Gateway Interface (Client) HTTP: HyperText Transfer Protocol JSP: Java Server Page 1 Proxy Server 6 2 Communication Web Server Server JVM 5 Servlet, SSI, Server 3 8 extension JSP CGI HTML programs images 7 applets scripts 9 4 DB-Server 4 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Server Components n WWW-Server n core component n provides static HTML pages, incl. embedded images, etc. (u, v) n provides Java applets, which may access a DB either directly (y) or via a communication server (z, {) n invokes server-side extensions (w) n invokes CGI programs (w) n invokes Java servlets, Java Server Pages (JSPs) (|) n delivers results of DB interactions (CGI programs x, Java servlets }) as dynamically generated HTML to web browser 5 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Server Components (2) n DB-Server n manages application data n may manage static HTML pages (or fragments) n Proxy-Server n caches results (HTML documents, images) of an HTTP request to improve response time for static information requests n dynamically generated or specially marked documents are not cached n Communication-Server n can be used to support DB-Server connectivity for Java applets (z, {) n Application-Server (not included in our chart!) 6 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Core Technologies n Client-side n Java Applets, Javascript n Server-side n Common Gateway Interface (CGI) programs n Web Server API n Java Servlets n Server-side Includes (SSI) n Java Server Pages (JSP) 7 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Client-Side Approaches n Goal: application-specific, dynamic clients integrated into the web browser n Capabilities n application modules can be downloaded (at run-time) to the client and executed n don't need to be pre-installed prior to invocation n module can access application server or DB server n may result in performance benefits n module can manage state across multiple interactions 8 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Java Applets n Application component embedded in HTML page (similar to images), stored on the web server n Dynamically transferred to the client to be executed in a Java-enabled web browser (u, v) n requires JVM integrated into web browser or loaded by Java plug-in n Applets can use full Java language support n JAR files (Java ARchive) can be used to package all class files needed by an applet for download over the network n General security restrictions (untrusted applets) n no access to local resources n network communication restricted to server of origin n Signed Applets n Security concept for applets (since JDK 1.1) n JAR file contains digitally signed applet files and digital certificate n guarantee that applet has not been modified after signed by providing party n client may trust applets, grant permissions based on certificates n Can be stored persistently on the client side 9 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Applet-based DB Access n Using JDBC/SQLJ n Type 3 driver n improved security n communication server for load balancing n works with unsigned applets n communication overhead (longer response times) n Type 4 driver n direct communication n requires either signed applets or identical DB-server and web server location n Additional alternatives and APIs n ODMG Java-Binding n for access OODBMS n defines language binding for ODL (Object Definition Language) to Java n API for executing OQL(Object Query Language) statements and processing of results n other Java-DB APIs n proprietary, DBMS-vendor-specific Java-APIs n applet implementations not portable 10 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Interaction with Application Components n CORBA n since CORBA 2.2: Java Language Binding, supported by numerous ORBs with Java support n Java-IDL n CORBA-compliant ORB, can communicate with server objects and server-side ORBs using IIOP (Internet Inter Orb Protocol) n available in all Java-enabled browsers as part of JDK 1.2 n avoids downloading CORBA runtime n Java Remote Method Invocation (RMI) n interoperability with CORBA/EJB n RMI over IIOP 11 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Applet-based Architecture for Web-IS persistent (applet) Web Browser program cache (Client) HTML ORB HTTP Web Server images applets IIOP Type 3 Communication Server CORBA ORB application objects Type 4 DB-Server 12 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Evaluation: Advantages n Enhanced UI-support n HTML only supports presentation of alpha-numeric data, potentially in tabular form n applets can leverage Java to process data and visualize complex data structures (e.g., geometry/CAD data) n complete UI has to be implemented in Java n transient storage of state within applet, across multiple user interactions n Connectivity, transactions n applet may connect directly to n DB-Server (y) n Connection-Server (z, {) n Application-Server n applet state can preserve DB-connections across interactions n long, multi-step transactions n distributed transactions 13 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Evaluation: Disadvantages n Loading time n higher initial loading time due to downloading applet (application logic, UI) from web server n solution: persistent program cache n in combination with signed applets n applets can be held persistently at client n alternative: use of Java interfaces, delaying download of implementation n Java security n unsigned applet can only connect back to server of origin n web, DB/connection server have to reside on the same machine (-> bottleneck) n this restriction can be avoided by using signed applets and appropriate client security policies n No adequate support for combined client/server-side transactions n passing transaction context to server-side web components not supported by HTTP n Requires enabling/allowing connection to DB server systems from web clients n may be suitable for the intranet n questionable for internet scenarios (security, firewalls, …) 14 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Client-Side Processing n JavaScript n object-oriented scripting language (syntax similar to Java) n can be embedded in or referenced from within a web page n script can interact with the Document Object Model (DOM) of the web page n can manipulate the page/HTML n can react to user events n Web browser n needs to support the JavaScript language n interprets the script elements n Main use n implement user interface logic of a web page n validate web forms n make web pages more interactive, dynamic and responsive 15 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems Ajax n Originally a shorthand for „Asynchronous JavaScript and XML“ n Indicates a set of technologies used for web applications n presentation based on XHTML, CSS n dynamic display and interaction using Document Object Model n data interchange and manipulation using XML, XSLT n asynchronous data retrieval using XMLHttpRequest n JavaScript (binding everything together) n Ajax „variations“ n use of another scripting language (e.g., VBScript) n data interchange based on JavaScript Object Notation (JSON) or others n Main benefits n avoids reloading complete pages, only interacts with server for subset of the content or avoids interactions altogether n user perceives web application as faster, due to asynchronous loading of data n state preserved inside JavaScript variables across multiple interactions n main container page is not reloaded 16 © Prof.Dr.-Ing. Stefan Deßloch Middleware for Information Systems